1. Field of the Invention
The present invention relates to a low-temperature regenerative type refrigerator to be used for cooling at an ultra-low temperature, e.g. MRI.
2. Description of the Prior Art
FIG. 1 is a construction diagram showing a single stage type Gifford McMahon cycle refrigerator which is a kind of a conventional low-temperature regenerative type refrigerator as disclosed, for example, in Japanese Patent Publication No. 10255/71. In the same figure, a displacer (a movable member) 2 is provided within a cylinder 1 into which is charged a cooling gas. A regenerator 3 is fitted in the displacer 2, and it is constituted by a phosphor bronze mesh or lead balls. The displacer 2 is reciprocated vertically in the figure within the cylinder 1 by means of an operating rod 4 extending through the upper portion of the cylinder 1.
By such movement of the displacer 2 there are formed a room temperature space 5 and an expansion space 6 within the cylinder 1. A seal 7 is mounted on the outer peripheral portion of the displacer 2 to provide a hermetic seal between the spaces 5 and 6. The displacer 2 is provided with gas passages 2a and 2b for communication between the space 5 and the regenerator 3 and between the space 6 and the regenerator 3, respectively. An expansion unit 8 is constituted by the cylinder 1, displacer 2, regenerator 3, operating rod 4 and seal 7.
A compressor unit 10 is connected to the normal temperature space 5 through a pipe 9. The compressor unit 10 comprises a compressor 11 for compressing gas which is exhausted from the cylinder 1, a water cooling type cooler 12 for cooling the compressor 11 using a cooling water 12a, a low pressure-side surge tank 13, a high pressure-side surge tank 14, an exhaust valve 15 and an intake valve 16.
The operation of such conventional refrigerator will now be described. Gas (e.g. helium gas) compressed by the compressor 11 is fed to the high pressure-side surge tank 14. If the intake valve 16 is open, the gas in the tank 14 flows into the room temperature space 5 of the expansion unit 8. The gas which has thus entered the space 5 passes through the gas passage 2a, then through the regenerator 3 which has been cooled in the previous cycle, whereby it is heat-exchanged (cooled), thereafter the gas thus cooled passes through the gas passage 2b and enters the expansion passage 6. At this time, there is no fear of the gas flowing directly between the spaces 5 and 6 because the seal 7 is provided on the outer periphery of the displacer 2. The gas thus entered the expansion space 6 expands and generates low-temperature heat to cool an article to be cooled (not shown).
Thereafter, the gas passes reversely through the regenerator 3 to cool the regenerator and reaches the room temperature space 5. Also at this time, the gas will never flow directly between the spaces 5 and 6. This exhaust-side gas passes through the exhaust valve 15 which is in an open condition, then reaches the low pressure-side surge tank 13 and is again compressed by the compressor 11. The compressor 11 is cooled by flowing the cooling water 12a into the water cooling type cooler 12.
When the conventional low-temperature regenerative type refrigerator constructed as above is operated at a low ambient temperature, the temperature of the gas flowing into the expansion unit 8 also becomes low because it is apt to be influenced by the ambient temperature. As the temperature of the gas flowing into the expansion unit 8 thus drops, the seal 7 shrinks and its function is deteriorated, resulting in that it becomes easier for the gas to leak and there occurs convection or deflecting flow within the regenerator 3, thus causing a marked deterioration of the heat exchange efficiency. Consequently, the no load temperature (the temperature of the expansion space 6) as the low-temperature regenerative refrigerator rises and the refrigerating capacity is markedly deteriorated. FIG. 2 shows the results of an experiment conducted to check the influence of the intake temperature upon the no load temperature in this type of a low-temperature regenerative type refrigerator. Also from this figure it is recognized that the no load temperature rises as the intake temperature drops. Also when the cylinder 1 is installed in a place where the temperature of the seal 7 sliding portion of the cylinder becomes low, the seal shrinks and the function thereof is deteriorated, thus causing deterioration of the refrigerating capacity.